Persons who work in polluted environments commonly wear a filtering face mask to protect themselves from inhaling airborne contaminants. Filtering face masks typically have a fibrous or sorbent filter that is capable of removing particulate and/or gaseous contaminants from the air. When wearing a face mask in a contaminated environment, wearers are comforted with the knowledge that their health is being protected, but they are, however, contemporaneously discomforted by the warm, moist, exhaled air that accumulates around their face. The greater this facial discomfort is, the greater the chances are that wearers will remove the mask from their face to alleviate the unpleasant condition.
To reduce the likelihood that a wearer will remove the mask from their face in a contaminated environment, manufacturers of filtering face masks often install an exhalation valve on the mask body to allow the warm, moist, air to be rapidly purged from the mask interior. The rapid removal of the exhaled air makes the mask interior cooler, and, in turn, benefits worker safety because mask wearers are less likely to remove the mask from their face to eliminate the hot moist environment that is located around their nose and mouth.
For many years, commercial respiratory masks have used “button-style” exhalation valves to purge exhaled air from masks interiors. The button-style valves typically have employed a thin circular flexible flap as the dynamic mechanical element that lets exhaled air escape from the mask interior. The flap is centrally mounted to a valve seat through a central post. Examples of button-style valves are shown in U.S. Pat. Nos. 2,072,516, 2,230,770, 2,895,472, and 4,630,604. When a person exhales, a circumferential portion of the flap is lifted from the valve seat to allow air to escape from the mask interior.
Button-style valves have represented an advance in the attempt to improve wearer comfort, but investigators have made other improvements, an example of which is shown in U.S. Pat. No. 4,934,362 to Braun. The valve described in this patent uses a parabolic valve seat and an elongated flexible flap. Like the button-style valve, the Braun valve also has a centrally-mounted flap and has a flap edge portion that lifts from a seal surface during an exhalation to allow the exhaled air to escape from the mask interior.
After the Braun development, another innovation was made in the exhalation valve art by Japuntich et al.—see U.S. Pat. Nos. 5,325,892 and 5,509,436. The Japuntich et al. valve uses a single flexible flap that is mounted off-center in cantilevered fashion to minimize the exhalation pressure that is required to open the valve. When the valve-opening pressure is minimized, less power is required to operate the valve, which means that the wearer does not need to work as hard to expel exhaled air from the mask interior when breathing.
Other valves that have been introduced after the Japuntich et al. valve also have used a non-centrally mounted cantilevered flexible flap—see U.S. Pat. Nos. 5,687,767 and 6,047,698. Valves that have this kind of construction are sometimes referred to as “flapper-style” exhalation valves.
In some flapper-style valves, a projection or fin is positioned on an inner surface of the valve housing to reduce sticking of the flexible flap to an inner surface of the housing during use, for example, as a result of breath condensation.